Fracturing of subsurface formations

ABSTRACT

SUBSURFACE FORMATIONS ARE FRACTURED BY THE PROCESS INCLUDING CONTAINING WITHIN A BORE HOLE WHICH TRAVERSES THE SUBSURFACE FORMATION, A SLURRY WHICH INCLUDES AN INORGANIC OXIDIZER, AND AN EXPLOSIVE FUEL FUEL SUCH AS TNT;AND DETONATING THE SLURRY EXPLOSIVE TO YIELD A MULTITUDE OF FRACTURES EXTENDING RADIALLY FROM THE POINT OF DETONATION, WHICH FRACTURES ARE OPENED SUFFICIENTLY TO ALLOW THE PASSAGE OF FORMATION FLUID THERETHROUGH.

[72] Inventors Arthur M. Spencer; 2,316,596 4/1943 Kennedy 102/23 Gordon R. Dysarl, Dallas; Arthur L. 2,707,436 5/1955 McCool 166/299X Anderson, Richardson, Tex. 2,911,046 11/1959 Yahn 166/285 [211 App]. No. 839,264 2,930,685 3/1960 Cook et a1. 149/15 [22] Filed July 7,1969 2,966,855 1/1961 Barco 102/23X' [45] Patented June 28, 1971 2,988,143 6/1961 Seotty 166/299 [73] Assignee The Western Company of North America 3,075,464 [/1963 Woodle et al. 102/23 Fort Worth, Tex. 3,134,437 5/1964 Karpovich 166/299 3,208,521 9/1965 Holland et a1. 166/285X 3,456,589 7/1969 Thomison et al. 102/23 541 rnscrunmc or SUBSURFACE FORMATlONS P 8 Claims, 5 Drawing Figs Attorney-Richards, Harris and Hubbard [52] [1.8. (II .1 166/299, V w A a, 166/308 ABSTRACT: Subsurface formations are fractured by the [51] Int. Cl. E21b43/26 process including containing within a bore hole which travep [50] Field of Search 166/299, 5C5 the subsurface formation, a Slurry which includes an inch 308; 102/20 23 149M511 ganic oxidizer, and an explosive fuel fuel such as TNT; and

detonating the slurry explosive to yield a multitude of frac- [56] Reerenm Cited tures extending radially from the point of detonation, which UNTED STATES PATENTS fractures are opened sufficiently to allow the passage of formation fluid therethrough.

PATENTEU JUN28 IHYI SHEET 1 [IF 2 LZZIT TIJ FIGZ FIG!

INVENT ARTHUR L. M3350 GORDON R. DYSART ARTHUR M. SPENCER PATENTEDJuH28|sn 3587.744

sum 2 [1F 2 INVENTORS: ARTHUR L. ANMRSON GORDON R. DYS'ART ARTHUR M. SPENCER X M QQIMW ATTORNEY FIG.5

FRACTURING OF SUBSURFACE FORMATIONS This invention relates to fracturing of subsurface formations. In another aspect, this invention relates to a novel process of fracturing a subsurface formation with an explosive to thereby yield a multitude of open fissures within the formation.

Explosives such as nitroglycerin and dynamite have been conventionally utilized in wells producing petroliferous fluids, such as oil and gas, in an effort to increase the productivity of such wells. In these conventional applications, the explosive charge is lowered into the well to the level of the petroleum bearing stratum and detonated to thereby break up or shatter the immediately surrounding rock and to enlarge the diameter of the bore hole at this point. These processes have been utilized not only to increase the productivity of an old well, but to bring in a new well just after the bore hole is drilled.

While these conventional techniques have been useful in increasing the permeability of the stratum immediately adjacent the bore hole, they are generally ineffective for fracturing the subsurface stratum at relatively greater distances from the bore hole unless the charge is actually planted within the subsurface formation adjacent the bore hole. For example, such charges have been planted within the formation from a secondary hole which is drilled from the main bore hole laterally into the formation. When this latter step is accomplished, the resulting explosion will again only fracture the formation immediately adjacent the explosive charge.

Various hydraulic fracturing techniques have been developed which will fracture the subsurface formation to greater distances from the bore hole than obtainable by the use of the conventional explosives. In addition, the hydraulic fracturing techniques can be used in very deep wells under conditions of high temperature and pressure which are highly incompatible with the sensitive conventional explosives, such as, nitroglycerin and dynamite.

Even though these conventional hydraulic fracturing techniques result in improved fracturing of the subsurface formation at great distances from the bore hole, these techniques will not increase the permeability of the formation substantially uniformly from the bore hole, but will in general open the formation along existing planes of weakness. Basically, these hydraulic fracturing techniques cause a relatively large fracture to occur in a single vertical plane within the formation which is contiguous with the bore hole, and thus do not cause a uniform permeability increase within the formation. In addition, when utilizing the hydraulic fracturing techniques, it is usually necessary to force propping agents within the formation to actually prop open the fissures formed from the force of the hydraulic fluid.

Therefore, one object of this invention is to provide a novel method of fracturing a subsurface formation substantially uniformly around a bore hole within the formation, and to a greater distance from the bore hole than is obtainable with conventional explosive techniques.

Another object of this invention is to provide a novel method of fracturing a subsurface formation which is traversed by a bore hole in a substantially omnidirectional pattern from the bore hole, such that the resulting fissures formed not only extend deep within the formation but are open and permeable to formation fluids after the treating process.

A further object of this invention is to providean improved method of blast fracturing subsurface formations within deep "wells having high temperature. and/or pressure conditions therein. I

Still a further object of this invention is to provide a novel method of increasing the permeability of subsurface formations by utilizing blast fracturing and hydraulic fracturing.

According to one embodiment of this invention, subsurface formations are fractured to yield a multitude of permeable fissures which extend deep into the formation radially from the well bore by a process comprising: positioning a slurry explosive within a portion ofa well bore which traverses the subsurface formation, said slurry explosive comprising an inorganic oxidizer, and a high explosive fuel such as TNT dispersed in an aqueous medium; and detonating the slurry explosive to yield the resulting fractured formation, It has not only been discovered that the slurry explosive will fracture the formation in an omnidirectional and penetrating manner not heretofore known in the art, but the resulting fractures are open and permeable due to an unexpected effect produced by the slurry explosive.

According to another embodiment of this invention, the above-described slurry explosive is confined within the well bore by a method which not only allows substantially all of the explosive force from the slurry explosive to be absorbed by the formation to form the above-described fractures, but will yield a minimum amount of damage to a casing contained within the bore hole above the point of detonation. This method includes generally, sealing the space above the slurry explosive by positioning an umbrella transversely across the bore hole above said slurry explosive, filling a section above said umbrella with a hard particulate material such as pea gravel, positioning a cement plug above the particulate material, and then positioning a column of liquid stemming above the cement plug to a point preferably adjacent the top ofthe well.

According to a further embodiment of this invention the above-described fracturing technique is completed to yield a highly permeable fractured formation extending around the bore hole, and then a hydraulic fracturing fluid is forced through the permeable fissures formed by the fracturing step to further provide fracturing of the formation. By this technique, the hydraulic fracturing fluid will be exposed to any plane of weakness which is intersected by the bore hole, and which traverses areas distant from the bore hole but is intersected by said permeable fractures. This action will form new permeable fissures along the said planes of weakness.

This invention can be more easily understood from a study of the drawings in which:

FIG. I is a sectional view of a bore hole illustrating a process for depositing slurry explosives therewithin adjacent the subsurface formation to be fractured;

FIG. 2 is a sectional view of the bore hole of FIGURE 1 il lustrating an explosive placement technique of this invention;

FIG. 3 is a schematic illustration showing the effects of a conventional nitrofracturing operation, and a hydraulic fracturing operation upon a subterranean formation traversed by a bore hole;

FIG. 4 is a schematic view illustrating the effects of the blast fracturing process of this invention upon a subterranean formation; and

FIG. 5 is a schematic view illustrating the effects of the blast fracturing process of this invention coupled with a hydraulic fracturing process upon a subterranean formation.

Thus, it has been discovered that the use of a slurry explosive which contains an inorganic oxidizing agent such as ammonium nitrate, or sodium nitrate, and a high explosive fuel such as TNT dispersed in an aqueous carrier in a fracturing operation, will result in the formation of a multitude of open permeable fractures which extend radially from the point of explosion, and deep into the surrounding formation. The slurry explosive which is used in this invention can include the slurry explosive disclosed in U.S. Pat. No. 2,930,685. The slurry explosive generally comprises the slurry blasting agents disclosed in U.S. Pat. Nos. 3,113,059 and in Re. 25,695, but further containing high explosives such as TNT. The above referenced patents are hereby incorporated by reference into this application. Basically, the slurry explosive comprises an inorganic oxidizer selected from ammonium nitrate, the alkali metal nitrates, and the alkali metal perchlorates together with a high explosive fuel such as TNT, all carried within an aqueous medium. If desired, other fuels can also be present, for example, fuel oil and powdered aluminum.

Generally, the slurry explosive contains from about 1 percent to about 40 percent water, generally about 15 percent; from about 5 percent to about 60 percent high explosive including TNT, composition B, smokeless powder; from about 0 percent to about 30 percent aluminum and/or fuel oil; and 5 percent40 percent of the inorganic oxidizer as described above, for example, ammonium nitrate, sodium nitrate, borium nitrate, sodium perchlorate, sodium chlorate, and the like. In addition, the material can contain water extenders such as formamide, ethylene glycol, sugar, and molasses, and gelling agents. The slurry explosive can be of any suitable density, but it is preferred that it be of a density substantially the same as the density of the subsurface formation to be fractured.

Now referring to FIGURE 1, a method is disclosed for depositing the slurry explosive within a well bore. As illustrated, well bore extends through nonoil bearing formation 11, and into an oil bearing formation 12. As shown, casing 13 is cemented within well bore 10 and extends therethrough to a point within the oil bearing formation 12. Lower portion 14 of well bore 10 extends within oil bearing formation I2 below casing 13.

Delivery pipe 15 is extended within well bore 10, and a bed of slurry explosive 16 is deposited within lower portion I4. It is noted that the above-described 108 slurry explosive is compatible with both water and petroliferous fluid which may be present within lower portion 14 of well bore 10. In addition, slurry explosive 16 will not be activated or deactivated by the high temperatures and pressures which may be present within any conventional well bore. For example, slurry explosive 16 can be deposited within a well bore under conventional temperature conditions ranging from about 70 to about 500 F., and conventional pressures ranging from about 15 to about 20,000 psi.

During the depositing of the bed of slurry material 16 within the well bore, conventional detonation devices can be posi tioned as desired therewithin. For example, as shown in FIGURE 2, two detonation devices 17 are positioned within the upper and lower regions of slurry bed I6. The detonation devices can be conventional time-bomb devices, or can be electrically actuated devices connected to a wireline extending to the top of the well bore.

After an explosive charge comprising the bed of slurry explosive 1'6 and detonation devices 17 is deposited within lower portion 14 of well bore 10 as illustrated in FIGURE 2, the charge can be sealed therewithin as will be described in detail below. It is to be noted that slurry explosives can be deposited within the bore hole not only by the process as illustrated in FIGURE 1, but can be delivered to the desired position within the bore hole in suitable containers, for example, metal containers, and plastic bags. If desired, the slurry explosive can be dropped from rather great heights within suitable containers to fall to the desired position within the bore hole. Since the slurry explosive is not sensitive to such impacts, the dropping operation can cause a rupture of the container with no detonation.

In addition, it must be noted that any particular section within a bore hole can be fractured according to the method of this invention. For example, this invention is not limited to the fracturing of the portion adjacent the bottom of a well bore, but sections in the intermediate region of the well bore can be fractured by the process of this invention, by depositing the slurry explosive on a suitable plug which is positioned at the desired point in the well bore. The plug can be a cement body positioned on a column of stemming, for example, a column of water or petroleum.

After the slurry explosive containing the proper detonation device is positioned within the desired portion of the well bore, the slurry can then be detonated to produce an improved fracture according to this invention. However, it is preferred that the slurry be contained, or sealed within the well bore by the process as illustrated in FIGURE 2. As shown, umbrella 18 is positioned over the bed of slurry 16 within lower portion 14 of well bore 10. Any high strength material can be utilized as umbrella 17, for example, steel, or reinforced concrete. Next, a bed ofparticulate material 19 is positioned over the umbrella to provide a blast absorbing pad. For example, absorbent bed 19 can comprise a bed of pea gravel.

After the bed of particulate material 19 is positioned, cement plug 20 is formed thereover. As illustrated, cement plug 20 extends up within casing 13 and is of a sufficient thickness to contain the absorbent bed of particulate material 19 under the force of the explosion of slurry bed 16. Cement plug 20 can comprise a plug made of any conventional rapid setting well cement such as for example, the Portland-gypsum cement mix disclosed in copending application .Ser. No. 767,893, filed Sept. 9, 1968.

It is preferred that cement plug 20 be at least about twenty feet in thickness within the conventional well bore 10 for each ton of slurry explosive utilized in slurry bed 16. For example, when 3 tons of slurry explosive are utilized, it is preferred that cement bed 20 go up at least through three collars of the conventional casing pipe (60 to feet of cement). In addition, particulate bed 19 should be at least about 10 percent as thick as cement plug 20.

After cement plug 20 has been set within well bore 10, it is preferred that a column of fluid 21 be positioned thereover for at least 2000 feet, or to the surface of the well. Fluid 20 can comprise water, and/or oil and the like. It has been found that the resultant shock wave from the detonation of the explosive charge causes very high tension pulses at every interface along well bore 10, for example, the interface between the top of the cement plug 20 and casing I3. Thus, it has been found that when the fluid column is positioned over the plug, the shock wave from the exploding charge will be dissipated therein and result in little or no damage to the casing 13 at points above the explosion.

After slurry bed 16 is positioned within well bore 10 as illustrated in FIGURE 2, the mass of slurry is detonated by the above-described detonation devices to thereby cause the fracturing of formation 12. The use of the above-described slurry explosive by the above-described technique will result in a multitude of fissures or cracks 22 which extend radially from well bore 10 as illustrated in FIG. 4. In addition, the detonation of slurry explosive bed 16 results in secondary fissures 23 which extend from fissures 22. It has unexpectedly been found that fissures 22 and 23 are substantially open and permeable to fluids contained within the oil bearing formation 12. Thus, a self propping action results from this detonation which was not heretofore known in the art of explosive fracturing.

For example, referring to FIG. 3, the dotted line cavity portion 24 with the very short fractures 25 schematically illustrate the fracturing effect of a conventional explosive, such as nitroglycerin or dynamite upon formation 12. These conventional explosives have a very high detonation velocity and result in a very high initial shock pulse being transmitted to surrounding formations. This initial high shock pulse results in the actual shattering and powdering of the formation immediately adjacent the bore hole to form cavity 24, and in a relative few short open fractures 25. Thus, the formation 12 does not absorb this initial energy, but rather reflects the energy back into the cavity 24. As a result, the use of these conventional explosives such as nitroglycerin and dynamite has only been effective in fracturing the formation to a relatively short distance from the well bore. Therefore, the use of these conventional explosives in fracturing operations has generally been only effective in unclogging formations at points near the well bore, and in the bringing in" a newly drilled well.

Also, FIGURE 3 illustrates a typical effect of a conventional hydraulic fracturing operation. As the hydraulic fluid is forced within well bore 10, and against the periphery of the walls thereof, it will force open and fracture formation I2 along any existing plane of weakness to form a fracture such as 26. Typically the existing planes of weakness comprise elongated planes within the formation. Thus, if the well bore 10 does not intersect a plane of weakness within the formation, the conventional hydraulic fracturing operation is generally not highly effective. Additionally, as shown in FIGURE 3, fracture 26 will only efficiently drain the areas which are immediately adjacent thereto and will not uniformly drain the formation.

The slurry explosive as utilized in this invention has a lower detonation velocity than the conventional nitroglycerin and dynamite explosives, and has a detonation time from one-third to one-half longer than the conventional explosives. In addition, the slurry explosive bed 16 as loaded is generally about three times as dense as the conventional explosive. For example, a typical slurry explosive bed 16 can have a density of l7 pounds per running foot, whereas a conventional nitroglycerin or dynamite explosive has a typical density of 5 pounds per running foot. Also, the slurry explosive as described above can be loaded into wells and subjected to environments of extremely high temperature and pressure without either actua tion or deactuation; whereas, the same use would be impossible with the conventional sensitive explosives, such as nitroglycerin and dynamite. in addition, the loading of the slurry explosive within the well bore is not hazardous since the explosive is not substantially sensitive to shock; whereas, the loading of nitroglycerin and dynamite within the well bore is a conventional safety hazard.

Now again referring to FIGURE 4, a detonation of slurry bed 16 results in formation 12 absorbing substantially all of the force thereof over a relatively long time period. This results in the formation of multiple fissures 22 and 23 which extend radially and uniformly from the periphery of bore hole 10. These fissures extend into the formation to a distance not heretofore known in the art. lt has been found that the area immediately adjacent the explosive charge is not extensively pulverized and powdered similar to conventional explosive operations, but larger particles are thereby formed. Additionally, it has been unexpectedly found that fissures 22 and 23 are open and permeable to the fluids within formation 12. Thus, a self propping" effect occurs as the bed of slurry I6 is exploded and acts upon the formation. The resulting fractured formation is uniformly permeable to a relatively great distance into the formation around the point ofthe explosion.

The mechanism of this self-propping" action is not fully known at this time. However, it is believed that in addition to the relatively slow acting force transmitted to the subsurface formation by the slurry explosive which causes the formation to absorb substantially all of the energy transmitted thereto, and to thereby fracture the same, that small bits of the formation are blown into the'fissures being formed and thereby act as propping agents therein.

Well bore 10 can then be pumped to produce any petroliferous fluid flowing thereto from fissures 22. Additionally, it is within the scope of this invention to couple a hydraulic fracturing operation with the blast fracturing method described above. If it is desired to further permeate formation 12 after the above-described blast fracturing operation, a conventional fracturing fluid, for example, water, can be pumped into well bore 10 and forced into fissures 22 and 23. Fissures 22 and 23 extend across a greater area of the formation than the fissures such as 26 illustrated in FIGURE 3, which are produced from conventional explosive operations. Thus, it has been found that fissures 22 and 23 generally extend across several planes of weakness which are positioned adjacent well bore 10. The fracturing fluid flowing through fissures 22 and 23 will thereby communicate with the existing planes of weakness to form hydraulic fractures 27 as illustrated in FIGURE 5. These hydraulic fractures 27 will generally form in the elongated vertical and parallel spaced planes of weakness existing in conventional oil bearing formations. Thus, the fissures 22 and 23 formed by the detonation of the slurry explosive will provide an access to the parallel planes of weakness from a single point of entry (well bore 10). This operation will result in a relatively larger permeable area being formed from a single well bore than heretofore known in the art.

The following example is given for illustrative purposes only in order to better facilitate the understanding of this invention.

EXAMPLE A well within the Maljamar field in southeastern New Mexico was fractured with slurry explosive according to the techniques of this invention. The well was 4,034 feet deep and before treatment was producing a maximum quantity of 19 barrels of oil per day.

The slurry explosive utilized comprised:

30 percent by weight TNT 20 percent by weight aluminum particles 15 percent by weight water 34 percent by weight ammonium nitrate 1 percent by weight thickener The slurry was deposited into 34 cans which were substantially cylindrical, being 10 feet long and about 5% inches in diameter. The cans were lowered into the bottom of the well, and two time-bombs were positioned within the slurry explosive column.

The resulting 340 lineal feet of explosive was sealed in the bottom of the well by the sequential positioning of a steel umbrella such as 18 (FIGURE 2), 64 feet of pea gravel, and I08 feet of rapid setting cement. After this, a column of oil was positioned upon the cement plug and extended to substantially the opening of the well.

The charge of slurry explosive was detonated, and the plugs then drilled out. The well was not cleaned out, but was immediately put on production. The above-described blast treatment resulted in a greater than tenfold increase in the productivity of the well. The well has been producing from 216 to 283 barrels per day for several months following the treatment technique.

Based upon experience with conventional explosives, this tenfold increase was totally unexpected in that a conventional explosive detonated within the bottom of the bore hole could be expected to increase the productivity of the well a maximum of twofold. Other wells have been fractured with slurry explosives to yield similar results. Additionally, hydraulic fracturing operations occurring after the blast fracturing operation of this invention yield an unexpected increase in permeability.

While this invention has been described in relation to its preferred embodiments, it is clear that various modifications of this invention will now become apparent to one skilled in the art upon reading the specification, and it is hereby intended to cover such modifications as fall within the scope of the appended claims.

We claim:

1. A method of fracturing a subsurface formation which is traversed by a well bore comprising:

a. positioning within said well bore adjacent said subsurface formation a mass of slurry explosive consisting essentially of from about 1 percent to about 40 percent water, from about 5 percent to about 60 percent of a high explosive selected from TNT, composition B, and smokeless powder, from about 0 percent to about 30 percent material selected from aluminum and fuel oil, and 5 percent to 40 percent of an inorganic oxidizer selected from ammonium nitrate, alkali metal nitrate, borium nitrate, alkali metal perchlorate, and alkali metal chlorate; and a detonator for said slurry explosive; and

b. detonating said slurry explosive to thereby form multiple permeable fissures radiating substantially uniformly from said well bore into said subsurface formation.

2. The method of claim 1 wherein said mass is confined within said well bore under temperature conditions ranging up to about 500 F. and pressure conditions ranging up to about 20,000 pounds per square inch.

3. The method of claim 1 wherein said slurry explosive comprises particulate ammonium nitrate and TNT dispersed within water.

4. A method of fracturing a subsurface formation which is traversed by a well bore comprising:

a. positioning within said well bore adjacent said subsurface formation a mass of slurry explosive containing an inor ganie oxidizer and a high explosive dispersed within an aqueous carrier medium and a detonator for said slurry explosive;

b. positioning an umbrella support means laterally across said well bore over said mass;

c. depositing a bed of particulate material upon said umbrella support means and within said well bore;

d. forming a cement plug over said bed of particulate material; and

e. detonating said slurry explosive to thereby form multiple permeable fissures radiating substantially uniformly from said well bore into said subsurface formation.

5. The method of claim 4 further comprising prior to said detonating:

positioning a column of fluid within said well bore upon said cement plug.

6. The method of claim 5 wherein said column of fluid extends at least about 2000 feet above said cement plug.

7. The method of claim 4 wherein said cement plug extends linearly within said well bore a distance of about feet for every ton of said mass of slurry explosive, and wherein said bed of particulate material extends at least about one-tenth the lineal distance within said well bore as said cement plug extends therein.

8. A method of fracturing a subsurface formation traversed by a well bore comprising:

a. forming multiple permeable fissures radiating substantially uniformly into said formation from said well bore by detonating within said well bore adjacent said subsurface formation a mass of slurry explosive consisting essentially of from about 1 percent to about 40 percent water; from about 5 percent to about 60 percent of a high explosive selected from TNT, composition B, and a smokeless powder; from about 0 percent to about 30 percent of material selected from aluminum and fuel oil; and from 5 percent to 40 percent of an inorganic oxidizer selected from ammonium nitrate, alkali metal nitrate, borium nitrate, alkali metal perchlorate, and alkali metal chlorate; and thereafter b. introducing a fracturing fluid into said well bore and forcing said fracturing fluid through said fissures to further increase the permeability of said formation 

